I think Neocore has several reasons to keep them in the forging menu. People can still use them if they have old lower level seasonal shards in their stash - whatever you manage to accumulate during a season will be yours, so it's quite possible that many people have lower level seasonal shards that they might want to upgrade eventually.

Also, the console versions of the seasons are always a couple of months (~6 I think) late compared to the PC, so the interface updates for these shards are later used in the console seasons.

And then, it's probably more time consuming to update the forging interface every time a season ends to remove them than it is to just keep those old forging options in there. Maybe Neocore will just keep them there and simply do a cleanup after season 10 or something like that - it's probably not worth the effort to do it individually every couple of months. They have bigger fish to fry atm (King Arthur).

As for the drops, I'm doing another Viridian VC and the shards seem to be dropping normally again, so it might have been just bad RNG for me...

Only the corresponding shards drop in each season, so in this one you will only see the empyrean echo shards. The other ones you mentioned were part of the previous seasons and were retired from the loot table when those seasons ended.

On this note, I would also signal to @MARCOPOLOCS‍ the very low drop rate of the seasonal shards (and shards in general). I'm not sure if it's a result of the various items' (uther's tarots and empyrean seals, I think) drop rate rebalance before the start of the season that might have had unexpected impact on other similar drops, or something else.

Even with the The Archeotech Vault tarot card on (tier 3), I often go several missions in a row without a single shard drop. Let alone the seasonal ones, which are the only ones I am after atm, to stock up on. It seems to affect the VCs too - doing a full clear of the hardcore Viridian VC gave me just a handful of shards - definitely noticeably less than before.