We don’t need 20 man “flex” raids in Classic Plus.
40 man raids should be the default raid size for any main raid like MC, BWL, etc.
Just make 40 mans easy like was intended in Classic then guilds can flex down to 30 man, 25 man, or even 20 man if they are good.
This way the raids are approachable for novice or underpeforming guilds and they can take full 40 man raids while allowing better guilds to use less players to clear.
No need to make the raids harder or have any menus to pick which size raid you want.
Just make 40 man raids like MC difficulty or BWL difficulty and let guilds self-flex their raid comp to match their performance.
This allows the most flexible raids and allows the better guilds to concentrate loot to fewer members or do split runs to get their alts loot at the same time.
It’s a win win win win win situation.
No need for 20 man options in the game UI.