Yellow fever is only an example of a vaccination requirement. It is true that it is mainly less tourist places, but some have a fairly developed tourist industry. However, whether they are tourist places is not really the point - vaccination is seen as a Public Health issue. A Covid vaccination requirement may reduce the number of tourists in 2021, but by the 2022 season the vaccine will be widely available.
As a Public Health issue, the decision to be vaccinated is one for the individual, but no vaccine, no entry. The numbers of people that can't be vaccinated for medical reasons will be fairly small, and countries may consider that the risk is not worth taking to make any exceptions. (Particularly because the 'won'ts' will try to find medical reasons to become 'can'ts', and who is going to judge the medical reasons?). Entry into a foreign country is a privilege and not a right. So yes, vaccination could be de facto 'mandated' for international travel.
Once the requirement is established, it can be difficult to remove it - I guess that this comes when the countries involved (i.e. where you are coming from, and going to) stop vaccinating their own population as a matter of course, because the number of cases has reduced to an acceptable level, whatever that may be.