Example: Create a 2/10 Multisig. Destroy one of the 10 seeds. Do I now have the equivalent of a 2/9 Multisig?
If it is 2-of-10 multisig. 2 keys are needs to sign before you can be able to broadcast transaction, while the remaining are needed to setup your wallet. It can not become 2-of-9. You need all the 10 keys for wallet recovery.
[...] technically it reduces the security of the wallet and considering 2 out of 10 then losing 1, the security concern for the remaining available cosigners is negligible.
Could you please elaborate how losing a seed as long as the quorum is available reduces the security of the wallet?