- there are only 8 send slots per each track BUT it's common to have more than 8 FX tracks in a project (for different purposes)
- after creating the FX tracks (or by opening some project template with having FX tracks already set up) every newly added audio track has its send slots filled with the first 8 FX tracks
- if you have different FX-es for different types of instruments (at least, some special effects for vocals), you don't need all of them together on any track
- by having used all the sends slots, you can't add a side-chain from that track
My suggestion is simple: let's divide FX tracks into primary (up to 8 in a project) and secondary (limited only by the DAW). The idea is to fill the send slots of the newly added track by only the primary FX-tracks. Of course, there should be some possibility to mark an FX-track as primary.