If the parting between a talent and a company is amicable, I could see selling their merch being okay. To use an example from another corp, I'm sure fans of Sana wouldn't mind being able to buy merch of her during anniversaries or major events, even if she is no longer with Hololive. Especially if the talent was getting their share of the sales revenue. That raises interesting questions about who would get merch released and who wouldn't, so just not doing it at all seems easier, but it's a road they could try and I don't think it would automatically be a bad thing.
However, I very much doubt that Nijisanji is sending Doki her cut of merch sales. And why would you do a "Fuck Selen: The Stream" and tell everyone how awful you think Selen is, and then have your store selling Selen merch, as if you haven't pissed off Selen's fans and only have the Selen antis left. If you're going to try and make money off the IP that parted livers helped develop, maybe don't take them out back and publically shoot them first if you want to still be able to sell merch to that liver's fans.