I am a Canadian banned from traveling to the US; will I be allowed entry into Jamaica if the flight is direct from Canada?

Comments have included much relevant information but seem all to be taking for granted a key fact without actually stating it:

Canadian citizens do not require a visa to enter Jamaica.

The general requirements are the fairly standard (ibid):

All visitors are required to be in possession of a national passport or other acceptable travel document establishing nationality and identity, and bearing a photograph. The passport or other travel document must be valid for the period of intended stay in the island. Visitors are also required to show evidence of sufficient funds to support themselves during their stay and be in possession of a return or round trip ticket to another country to which they are admissible.

and specific considerations:

A person desirous of visiting Jamaica:
• Should be in possession of a valid visa unless exempted from this requirement under any of the prescribed regulations.
• Must be in a position to support himself/herself and accompanying dependents for the duration of stay;
• Should not be of unsound mind;
• Should not be the subject of a certificate submitted to the Immigration Officer by a health officer indicating that for medical reasons, permission to land is not desirable;
• Should not have been sentenced in a foreign country for any extraditable crime within the meaning of the Extradition Acts 1870 and 1906;
• Should not be the subject of a deportation order in force under the Aliens Law, Chapter 9;
• Should not be the subject of a landing prohibition order from the Minister of National Security;

The last three items concern criminality but all as recognised by Jamaican authorities, rather than US ones. Also all seemingly much more serious crimes than an assault 20 years ago. If Jamaica really cared a lot about lesser issues it could have included them above.

No visa required means no forms to fill in that ask embarrassing questions. If Jamaica does not know of your past it won't bar you from entry because of it.

So the Comments seem mostly to be considering, in effect, "What are the chances that someone has tipped off Jamaica about my past transgressions?"

My guess is next to no chance. While there is some international sharing of such information (eg Five Eyes as mentioned by @TimLymington) it seems unlikely there is Canada/Jamaica exchange. Two reasons for that (i) is that nobody seems to have heard of it and (ii) intelligence sharing is a very sensitive matter that by precedent can take decades between being mooted and implementation.

"nobody seems to have heard of it" might not sound indicative where 'it' is an intelligence sharing operation but nevertheless it is common knowledge that the following co-operate closely in some intelligence and co-operate in addition with many others for a more limited range of intelligence issues: Australia, Canada, New Zealand, United Kingdom, United States, Denmark, France, Netherlands, Norway, Germany, Belgium, Italy, Spain, Sweden.

Jamaica does not feature amongst the above.

In any case even if Jamaica is fully aware it has no obligation to prevent your entry just because USA has banned you. There is essentially no way to be certain other than post facto ie to "suck it and see". Turning up without a visa looks promising. You would save the cost and trouble of an airfare if you apply for a visa (as mentioned by @DJClayworth) and that is rejected but might not even if a visa is granted to you since there is always the chance of being turned away at the border (if not for an assault 20 years ago, for some other reason or even 'no reason'.)