Did Gautama Buddha endorse the caste system?

In my unbiased opinion Gautama Buddha did somewhat support and oppose the caste system.

Opposition:

• Anyone of any caste can join the order
• Anyone of any caste can become an arahant
• Laws of kamma apply to all castes

Support:

• States that people gain higher or lower births based on karmic causes
• Sammasambuddhas are always born as high caste (Kshatriya, Brahmin)

The Assalayana Sutta and many other suttas seem to oppose the caste system:

"No, Master Gotama. Even a noble warrior... Even a brahman... Even a merchant... Even a worker... (Members of) all four castes — if they take life, steal, engage in sexual misconduct, tell lies, speak divisively, speak harshly, engage in idle chatter, are greedy, bear thoughts of ill will, & hold wrong views — on the break-up of the body, after death, reappear in the plane of deprivation, the bad destination, the lower realms, in hell." (MN 93)

But then it's explained that Upali a chief arahant was born in a low-caste womb because he in a past existence as a Kshatriya had insulted a Buddha:

"Sneering at the Self-Become One, peaceful-hearted and attentive, today, due to that bad karma, I am born in this low-caste womb.

Don’t transgress even one moment; you will grieve for the moment missed. The moment is prepared for you: endeavor now for your own good." (Tha Ap 8)

The caste system still exists informally in countries where there's no formal caste system and the laws of kamma still apply...people are still viewed as like higher or lower caste automatically they still gain more fortunate or less fortunate births regardless.

But the caste system (whether formal or informal) only applies to lay people not to monks. A being who attains arahantship/enlightenment/perfection is beyond such a conception of any caste and is higher than non-arahants regardless.

Since the number of past existences is countless we've all experienced higher and lower existences many times before:

"When you see someone in a sorry state, in distress, you should conclude: ‘In all this long time, we too have undergone the same thing.’ Why is that? Transmigration has no known beginning. … This is quite enough for you to become disillusioned, dispassionate, and freed regarding all conditions." (SN 15.11)

"When you see someone in a good way, in a happy state, you should conclude: ‘In all this long time, we too have undergone the same thing.’ Why is that? Transmigration has no known beginning. … This is quite enough for you to become disillusioned, dispassionate, and freed regarding all conditions." (SN 15.12)

I don't think he necessarily endorsed it. It was just the cultural reality of the time... just as were the Buddha's statements regarding women. That said, take a look at any sutta having to do with Upali. He was born a sudra, became enlightened, and was asked to speak by Maha Kassapa at the First Council.

For more on this topic, check out the article Buddhism and the Caste System written by Y. Krishan.

No

He just shown the path of truth to all the human beings and he didn't care who is he teaching.
Why do you think that he just taught high caste people.

He had also teach Dhamma to killer like "Angulimala" what was his cast.
Once brahmins were prank with one toilet cleaner and then Buddha reach there and asking brahmins to accept that guy as a human but they (was joking) conversely ask Buddha to teach him Dhamma.
Buddha replied them it's his pleasure to do.

So many examples and if you'd think he had endorsed high cast like brahmin then Indian brahmins should be Buddhist but picture is different even they won't accept Buddha as it is.

Buddha declined veda because of caste system defined in veda, so why Buddha would endorse or would discover caste system.

Buddhism is secular so there's no point to even think if he endorse caste system.

He bring Dhamma to the life of all the human being who is suffering from everything.

The Buddha did not endorse the caste system. Instead, he taught that any person, regardless of their birth, is destined for good outcomes, if they are consummate in virtue and indulge in good practices.

From the Gihi Sutta (AN 5.179):

In the same way,
wherever one is born
among human beings —
noble warriors, brahmans,
merchants, workers,
outcastes, or scavengers —
if one is tame,
with good practices,
righteous,
consummate in virtue,
a speaker of truth,
with conscience at heart,
one who's abandoned birth & death,
completed the holy life
put down the burden,
done the task
fermentation-free,
gone beyond all dhammas,
through lack of clinging
unbound:
offerings to this spotless field
bear an abundance of fruit.

From the Sundarika Sutta (SN 7.9):

Then Sundarika the brahmin went up to the Buddha, and said to him: “Sir, in what caste were you born?”

“Don’t ask about birth, ask about conduct.
For any wood can surely generate fire.
A steadfast sage, even though from a low class family,
is a thoroughbred checked by conscience.

Did Gautama Buddha endorse the caste system?

Of course not. There's a whole chapter in the Dhammapada where it shows what qualifies one as a brahman. None of them use high birth or high caste as the criteria:

Whoever does no wrong in body, speech, heart, is restrained in these three ways: he's what I call a brahman. ~~ Dhp 391 ~~

About the Buddha being born in the high caste, we shouldn't blame Him for that. If any, the blame should be on the way folks treated one another in ancient India. It's simply a skillful mean to be born in whatever condition that is most conducive and effective in propagating the Dhamma. The Buddha was a Kshatriya and He already got all kinds of attacks from all kinds of religious sects. Now imagine being born an Untouchable in ancient India and trying to propagate a whole brand new religion, well, good luck with that!