What is a soft fork?
A change that tightens Bitcoin's rules, so nodes that haven't upgraded still accept the new blocks.
Because older software keeps working, a widely adopted soft fork doesn't split the network. SegWitSegWitSegregated Witness: a 2017 upgrade that moved signatures into a separate part of each payment, making room for more payments. and TaprootTaprootA 2021 upgrade that added Schnorr signatures and addresses starting bc1p, making complex payments cheaper and more private. were both soft forks.
Proposing one is easy. Switching one on needs very wide agreement among users, node operators and miners, and has usually taken years.