Multisig transaction types use standard CHECKMULTISIG opcodes (which have been present in Bitcoin since its initial release in January 2009) and P2SH addresses (standardized in BIP13 in October 2011). Both are very commonplace nowadays.

Consensus changes sometimes happen in Bitcoin, to add features or improve security. However, to the best of my knowledge, valid useful scripts have never been invalidated by such changes, nor do I expect such a thing to ever happen. Loss of money caused by a protocol change should and likely would cause an outrage.