What valid transactions are not broadcastable? Many valid transactions are not broadcastable by default in bitcoin core to avoid attacks that could harm the network. These would rarely be found in a node's mempool but could, however, be found in blocks because they don't violate any consensus rules.
What transactions fit these criteria?
The following transactions are not broadcastable with the default bitcoin core configuration but could be found in new mined blocks:

Transactions whose fee is smaller than 1 sat/vB (and RBF bumps that increment less than 1 sat/B).
Transactions with size greater than 100 vkB (and smaller than ~ 1 vMB)
Transactions with outputs amounts smaller than 566 satoshis (dust limit)
Transactions that are considered non-standard (I will try to list them because this is a very broad category)
Transactions with scriptSigs larger than 1650 B (and smaller than 10kB)
Transactions with more than one OP_RETURN output or an OP_RETURN output bigger than 83 B.
Transactions with scriptPuKeys of bare n-of-m multisigs, where m &gt; 3.
Transactions with scriptPubKeys or ScriptSigs that make the transaction malleable.