How long are Paris Museums open after last entry

https://www.musee-orsay.fr/en/visit

Check the visiting hours page for each museum, they're usually very clear. The Musee d'Orsay says:

Last admission to the museum at 5:00 p.m., last admission to
exhibitions at 5:15 p.m., galleries close at 5:30 p.m. Late night
opening on Thursdays until 9:45 p.m. Last admission to the museum and
exhibitions at 9 p.m., galleries close at 9:15 p.m.

I don't know about the Orsay Museum, but it's extremely common for European museums to start chasing people out at least 15 minutes beforethe advertised closing time. The farther you are from the museum exit, the earlier you will probably be approached by a staffer. You may be able to dally a bit, but I generally assume I'll lose up to 30 minutes at the end of the day. That's for rather large museums; small places won't rush you as much at the end of the day, because they know it won't take very long for you to get to the door.

You can usually stay in the shop until much closer to closing time.

Very true on getting shooed out of a room early. Especially the Louvre.

The Louvre rolls up the museum a half hour before closing pushing people out of galleries and closing them. Everyone is out of the museum on the street or in the mall by closing time. There is no lingering after the closing hour. I would just delete the last half hour when planning as you will not be able to enjoy the art while you are being herded out. -- smaller museums may start the roll 15 minutes ahead -- but they all have everyone out at closing.