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They retain their full readable capacity... but in most places they are used the data changes.
i.e. within a cache layer the algorithm applied might be LRU (Least Recently Used), which means to put new items into the cache the oldest items are evicted. Eviction here just means over-written... but if you can't write, then for the purpose of cache your storage is reduced as you can't add new items to the cache.
Example on LFGSS... the attachments. The attachments live in an LRU cache, and new attachments in hot (most visited) conversations will be requested the most. So those items are added to cache, and because cache is always full (it's expensive SSDs and less capacity than long term storage) the least recently served (oldest) item in cache is deleted from cache to make space for the new item. If I can never delete the oldest, then eventually I can never add newest, making the cache server useless to me (because requests would be cache misses and reach the far slower spinning disks somewhere... I care about speed, as slower reads take longer, resulting in contention, which floods networks - and networks are the problems cloud providers really have. Fast caches, SSDs, save networks.
Velocio-
Thank you for the detailed answer. I have experienced some sudden SSD failures so realised they don't give you the time to react that HD are inclined to.
You mentioned more than one network and different sites in your first reply and that's partly why I was asking the question as I've recently built a multi site multi network installation for my home/office/workshop setup which involves a 5g bridge and consequently I've been feeling that I'm closer to an ideal situation regarding backup than ever before. Basically data is being backed up off site so I'm wondering whether cloud backup is something I should still be considering.
I'm probably not your average user as I've professional experience in IT dating back to the mid 80's so it was great to have a detailed response. Especially underlining what I had already found through my 10 years experience of SSD failures, they are by no means safer than HD but most users seem to think they are infalible.
Airhead
As to how they break: Their memory can only take a number of writes before it fails... enough bits failing and the SSD can no longer find places to put files. So they degrade whilst bits are failing
This seems to suggest the total capacity of the drive would "shrink" the more it's being used.
I always thought they retain full capacity ..but then just go die at some point and that's that.