BCCI has not allowed active Indian cricket players to participate in foreign T20 tournaments, even the ones associated with the IPL franchises.

Will they allow this in any year before 2028?

As of 2024, players who retired from IPL and international cricket for India are able to join foreign tournaments -- this question only concerns policy related to active IPL and/or India players. So, for example, someone who is an active IPL player in one year, then plays in another T20 league elsewhere, then plays in the IPL again the following year, all fully allowed by BCCI and not in any gray area. And not requiring them to 'retire' then 'unretire' or any nonsense like that.

Most foreign T20 leagues would count, and other limited-overs tournaments may also count.