At the end of the season, the team, regardless of whether they win the championship, are splitting up.
I thought this was perfectly fine, although perhaps saying "the team members" would have been nicer (in my opinion). However, apparently, it should be rewritten as
At the end of the season, the team, regardless of whether they win the championship, is splitting up.
But this bothers me a) because I thought "team" could be both singular and plural depending on the situation, and b) with the clause "regardless of whether they win the championship" where "they", a plural, is used, wouldn't it be odd to use "... is splitting up" just to match "team" which is apparently singular? (That's why I chose to keep "team" plural in my first attempt at editing.)