The main clause or "apodosis" ("it would be better") is the main clause of a type 2 conditional.
The if-clause or "protasis" ("if this thread is now closed") is the if-clause of a type 1 conditional.
This kind of "mixed" conditional is common in spoken English and less formal kinds of written English (on webpages, in emails, etc.).
It can happen when a speaker begins his utterance in a conjectural or tentative frame of mind ("X would be better"), but then focuses on the immediate situation ("if Y is done").