I know of 2 instances where a max level character might require a lower level alchemy solvent that is unobtainable to them (for something that is relevant to them at their level). I've submitted /feedback on these but they have not been fixed (and I can't imagine either of these are particularly difficult to change).
- Orzorga's Red Frothgar is a drink that scales up to CP160, but requires Clear Water to craft. As far as I can tell, Clear Water is unobtainable to max level characters with max alchemy rank. (And no, I am not pay 10k to reset all my skill points just to harvest Clear Water). This also seems to be the only food item whose ingredients become unobtainable once your character progresses (if anyone knows of any others, please list them and I'll add them here).
- Max level alchemy writs very rarely seem to require Cloud Mist to complete. As far as I can tell, Cloud Mist is unobtainable to characters who are actually doing max level alchemy writs unless they happen to have the right CP.
For the writs, the fix is obvious: replace Cloud Mist with Lorkhan's Tears. I would recommend doing the same for Orzorga's Red Frothgar, but I don't believe the developers will remember to update it if they ever increase the level cap. I'd rather see Clear Water replaced with something like Isinglass (though Lorkhan's Tears is fine too).