Entering raids on Normal, Heroic*, Mythic* has always been an option. *If available
To enter old expansion raids on LFR difficulty you must queue at an NPC, this means having 2-4 instances added to your lockout per raid instead of 1 raid.
Possible solutions:
Legacy Dungeons + Raids do not increase your instance limit.
Instance Options → Raid Difficulty → LFR (Legacy) ie not current expansion.
LFR NPCs offer a multitude of queue options for the same raid, condense this to 1 option per raid.
Request:
Add an indicator for which raid the LFR queue option applies to. ‘Destroyer’s end’ is not a boss name nor raid name - it is difficult to understand which raid you are entering when queueing old LFR.