The system to prevent accidental gamemode switching is still in effect even after the player got a blackout (see screenshot). Since the player has nothing more to achieve after getting a blackout, it should be possible for them to switch gamemode without it being prevented.

Notes:

• Add tag bingo.spectator in bingo:item_detection/goals/announce_blackout
• Check team in bingo:tick/handle_player_join and if 25 items are gotten, add tag bingo.spectator

• Maybe add another information in tellraw in bingo:item_detection/goals/announce_blackout like

  Note: From now on manual gamemode switching with shortcuts is enabled, so feel free to change to spectator mode and wander around.

  so that people know about it

I don't see the necessity. It's something they are either gonna notice or not, and it's not a big deal if they don't.

I guess I could put it into the tooltip explanation in the preferences. I just don't want to spam chat with even more info, when I can avoid it.