Obama never tried to revoke visas while the people holding them were in the air.

I think there can also be a big shift in who is given a visa, without there being much visibility. Time will tell. That's not part of the entry procedure but is part of border policy more generally.

I really try to avoid political discussions on HN, as I dislike both of the major parties. I would suggest you read a book called 'Operation Troy', what Obama did IMO is much worse than revoking a visa, he extrajudicially killed an american citizen with a drone strike not in the field of battle.

You avoid political discussions by bringing up a completely unrelated subject that attributes negatively to an ex president?

So if I read a book about Obama abusing the power of the US presidency, how will that impact my thinking about whether the approach to border policy has changed under the new administration?

It's like you haven't been reading the news for the past four months. So much has changed, and it is so much of an anything-goes clusterfuck right now that it's hard to keep up.