In pratice there is only the Riot.im/Element.io Matrix.org homeserver to Libera.chat IRC server bridge. And that bridge regularly disconnects people for being idle because storing and transferring state is so heavy.
The reason for this limitation is actually on the IRC side - we can only have so many users connected via Matrix, so we need to time out stale ones.
I asked around and it seems like you're right about it not being a resource issue (network or computational). The issue is that the bridge is unstable and having tens of thousands of matrix.org users join/parting every now and then disrupts the IRC channels.
It's also mentioned in Libera's FAQ:
> At our request, the EMS-hosted Libera.Chat bridge regularly prunes idle connections to minimize disruptions to IRC channels during bridge restarts.