Me and some old friends (i.e we also played together back in the 80s) have been playing an AD&D campaign for several years now. Level drain is scary for players, but that is one of the main differences between old-school gaming and the more modern style of play and I think it makes the game exciting (I play and DM).

What you could do is make the Restoration spell more cheaply/easily available at a temple which the character worships (or to which the party's cleric belongs). Also the high level cleric might take magic items as well as gold. That way level draining undead are still a real threat and will derail the party's gaming, take them out of the dungeon, use up resources etc.. All depends on what kind of game you play though - everyone's campaign varies (that's why they have such calm discussions on Dragonsfoot!).

Oh, also we keep Ghost ageing too - although if I was going to house rule something I might be tempted to do that - because (as happened in 2nd Ed modules more) Ghosts are really great as plot devices or creatures the characters interact with, rather than merely another monster to fight. And their ageing effect interferes with that. Also if the party has been gaming a while (you said 10th level), a higher level character may have a Potion of Longevity stored away in a vault in their tower/keep. Or indeed at that level, they could set up a laboratory and make one; one wizard character in our campaign is wasting huge amounts of gp trying (with some limited success), to do just that. As a rule I tend to keep monster attacks as is, but provide campaign remedies which cost gp & items and take the players time (and might even lead to other encounters).

I agree with the above solutions - I've rearranged most of the spells for balance. But at 10th level, should they not have access to protective and restorative magic on their own? That is, the power of higher creatures is offset by the characters having more means to mitigate the dangers.

Also a 4th level character adventuring with 10+ level characters- if the latter take measures to protect the lower level character like they would a henchman, the 4th level character will rocket up in levels, catching up.

What edition do you play? Some of this is also expectations - for 1E, I lay the expectation that characters will die all the time, and so don't get emotionally connected to your character and get used to starting at level 1.

These days my gaming tends to be in the literary style - in other words I try to create the feel of the Appendix N et al books and stories. I tend to have fewer encounters, shorter adventures, no mega-dungeons, etc.

As such, something catastrophic requires something special to counter it.  Levels drained?   Now you need to go an a quest for the Maguffin of Restoration.  Make that a challenging enough adventure and when the levels are restored they feel they have earned it.

I do similar with death.  I tend to have little or no resurrection magic.  When an Elf character recently died in battle in The Lichway (in an OSE B/X campaign) I slightly modified the Dolmenwood module Winter's Daughter.  The other Elf in the party had a dream that the dead character was being held in Elven limbo by the ice queen.  They were then found and brought out, and thus back to life, when the party successfully completed that module. This "resurrection" ended up being much more fun and memorable than paying for a spell at the local temple.

SMC