A transaction with a 78 bytes OP_RETURN is refused, however with shorter information (e.g. 74 bytes) is accepted, though as much as 80 bytes ought to work.

What’s the protected restrict for use?

Per these references the utmost size is 80 bytes (83 for complete script) (since v 0.11):

https://stackoverflow.com/questions/24845429/maximum-size-of-data-bitcoin-op-return-tx-can-handle
https://developer.bitcoin.org/devguide/transactions.html#null-data

The under transaction produces error: {"error":"-26: scriptpubkey"}.
Related transactions with barely shorter OP_RETURN are effective.

01000000000102ae5c4a7139f0f47cf4bf431d99f110e73d2cd7e03faae56d56d7c09db3ee3c400100000000fbffffffc8ab005126b7b871d7c9621789de87282f4cd13e851fd286b0e5aae02e21c0b90000000000faffffff0350c30000000000001600146f26d2633d5307d9c4b1ca5b4c282201bae4ab144e15000000000000160014bb5bb9b9a07718bd0bfeef385f6a184787b584da00000000000000004f6a4d535741503a424e422e425553443a626e62317368366e757a7474336463657679346e676d7a746b7061706e76787179376a653074307564723a3635383137323831393a74656d7963683a3330300247304402200ca03ef0f249471dbee806b0961892c7141339460a5890479fdbebd3ec06ce8d02201a2275d54849f660ba01cd2799c0ac57a4bfca6d21115ddeecf63ebfd71898df0121034df96f5dacf5e22fb8611e6863e48f6777c7e96064c81196e23dcefd35017d0e02483045022100987199ff71731d44520bc1ed3704a6a66725ea890479ea5078b6b89ebb854d8402202dd52791a28c9d28c1672785765aab811b1549a1ec031f7c15a4ee472e70c4200121034df96f5dacf5e22fb8611e6863e48f6777c7e96064c81196e23dcefd35017d0e00000000

{
    "data_hex": "535741503a ...",
    "script": "6a4d535741503a ...",
    "script_type": "null-data",
    "worth": 0
}

Notice: use case is consumer for thorswap.finance cross-chain DEX.