A hard fork is less strict rules, where previously invalid becomes valid. Forces upgrade to remain operational.
A soft fork is more strict rules, where previously valid becomes invalid. Does not require upgrade although failure to upgrade can lead to less secure or less functional nodes.
This is backwards, soft forks are acceptable only because your UTXOs prior to the soft fork can still be used even after the majority of the network upgrades.
We want people running node software from 5 years ago to be on the same network.
Soft Forks Introduce More Restrictive Rules:
A soft fork tightens the rules of the blockchain protocol. For example, a soft fork might introduce a new rule that limits the size of the block or the structure of certain types of transactions that weren't previously restricted. These new rules only apply to transactions or blocks created after the implementation of the soft fork.
Why Existing UTXOs Remain Valid:
The key point is that the changes introduced by a soft fork are designed not to retroactively invalidate the existing UTXOs or the fundamental ways that blocks and transactions are processed by older nodes. The rules for validating transactions that were already in the UTXO pool before the soft fork remain unchanged for those transactions. Therefore, these UTXOs can still be spent under the old rules, even though new transactions must comply with the new, stricter rules.
Illustrative Example:
Imagine a soft fork that introduces a new rule where all new transactions must include some additional verification data that wasn't required before. Nodes that have updated to the new protocol will check for this data and reject transactions that don't comply. However, transactions that were formed and included in the UTXO pool before the fork don't need to meet this new requirement—they're still valid because they were valid under the rules that were in place when they were created.
Continued Node Compatibility:
Nodes that have not updated to the new version of the software will not be aware of these new rules. They will continue to see blocks that conform to the old rules as valid, so long as those blocks do not explicitly break any of the old rules. Because the new rules are strictly more restrictive, all blocks created under the new rules are also valid under the old rules, thus maintaining network coherence.
hopefully that makes it more clear. Soft fork == more restrictive rules, but UTXOs and blocks created before the fork as still valid, that's part of how a softfork works, everything created before the fork are grandfathered in. If you don't believe me I strongly recommend you do some more research into exactly what a soft fork does.
