Must airlines warn passengers about changes of required travel documents?

NO.

It is the responsibility of the traveler to make sure they are compliant with the current, day of travel, regulations for their destination.

This has always been the case and will not change.

For future travel, you can easily verity the rules the airline will use be visiting TIMATIC provider.

The person ultimately responsible for ensuring a traveler’s paperwork is in order is the traveler him- or herself. This goes for the validity of the passport or other travel document proving identity, the securing of visas or other authorizations, the meeting of health requirements such as immunizations and obtaining documentation thereof, the securing of any required financial or insurance coverage and the documentation thereof, and all other requirements for taking the flight and presenting oneself for entry at the destination.

The airline is interested only in selling you a ticket to fly to the destination and in avoiding having to fly you back because you do not meet the requirements. As such, they will do a quick document check when you check in, but they would never assume the liability for keeping track of everyone’s entrance requirements prior to that. They are not required to take on that liability by EU regulation, and I am not familiar with any national laws that would stipulate the same.

They do not hide this. You haven’t told us which airline you were planning to fly, but Emirates’ visa requirements page, for example, is typical, and explicit:

Remember that visa requirements can change with little or no notice. Please double-check foreign entry requirements for travellers from your country/territory of nationality by contacting the local embassy or consulate of your destination country/territory.

Please note it’s your own responsibility to obtain a visa when required and we accept no liability for your failure to do so. You can learn more in our terms and conditions.

This information is provided by an external third party, and we are not responsible or liable for the information contained on this website. If you hold a non-standard passport – such as a diplomatic passport – you should contact the local embassy or consulate of your destination country or territory to learn about any specific requirements.

Lufthansa is similarly direct:

It is each passenger’s responsibility to find out about current entry and exit regulations and to take the required documents with them. Lufthansa accepts no responsibility in these circumstances. Passengers are also obliged to obtain all the necessary travel documents and visas for themselves and their children as well as to comply with all the regulations of the countries that they will be flying over, to or from. Of course, this also applies to our regulations and instructions in this respect.

Travel documents and visas must be valid for the entire duration of your journey, including any breaks in travel. We do not accept any responsibility in this regard; in particular we are not obliged to check the validity of documents. We are not liable for the consequences if passengers have not acquired the necessary paperwork or do not comply with regulations or instructions.

The airline Conditions of Carriage (i.e., the contract with the airline you agree to when you purchase a ticket from them) will mostly likely explicitly disclaim liability as well. In the case of Emirates,

3.1.5 We will not be liable to you if you do not have all passports, visas, health certificates and other travel documents needed for your journey or if any of those documents are out of date or if you have not obeyed all laws, regulations, orders, and other requirements of all countries you will exit, enter, or through which you will transit during your journey.

In the case of Lufthansa,

13.1.1. You are responsible for obtaining all required travel documents and visas and for complying with all laws, regulations, orders, demands and travel requirements of countries to be flown from, into or through which you transit. Further, it is the passenger’s responsibility to ensure that their travel documents and visas will remain valid for the entirety of their travel, accounting for the possibility of flight disruptions. Lufthansa is not liable for the passenger’s failure to take reasonable precautions to ensure that their travel documents and visas remain valid.
13.1.2. We shall not be liable for any and all the consequences, however remote, to any passenger arising from his or her failure to obtain and carry with them necessary valid travel documents or visas or to comply with such laws, regulations, orders, demands, requirements, rules or instructions.

It would have been a nice customer service gesture if the airline had informed you and other passengers with tickets for Namibia of the change, but they were not legally required to, and chose not to do more than the legal requirement in this case. I know this is not the answer you were hoping to hear, but unfortunately, you have no recourse here.