You're probably right.
That said it's quite difficult to say without knowing what kind of legal procedure route they went (bankrupcy/insolvency/cessation of activity and what not).
Could be international rebranding but I doubt it for various reasons (could be wrong tho). Internal dispute (and amicable settlement) seems plausible, someone have to check organizational chart/shareholder. I really don't think it's about it, but I wonder if their demo still use Unity or their inhouse engine.
My guess is cessation of activity. If it was insolvency or bankruptcy they most likely would have left the titles up for sale instead of delisting everything, so they could generate money for creditors. We probably would have heard about a sale of IP as well.
The
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says "ILLUSION has no successor company" but the X account for illgames was
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with updates for Honey Select 2 and AI Shoujo. They basically nuked all the posts at the beginning of the year.
Whatever is going on, it seems sketchy.