What valid transactions are not broadcastable?
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.