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.
As long as metadata is available then there is no problem with the wallet recovery even if one of the seed phrases is lost. to OP, 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.