Well it has. IFAB laws say a player that’s sent off “must leave the vicinity of the pitch”, coaches must “leave the technical area”, they’re two different things.
Wenger was sent off in 2009:
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I don’t remember the Pearce one to be honest but he left us in 2007 so it’s either always been the case (and that decision for him to not go in the stands was wrong - which is likely seeing as he’s thick as pig shit) - or that’s it’s been the case for at least 14-15 years, in which case the other poster should have probably noticed that managers don’t have to go to the dressing room in the intervening period.
More examples:
Conte last season:
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Pep sent off:
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Mourinho Sent Off:
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So erm yeah, I’m quite surprised the fact he’s confused about TT not going into the dressing room.