You can basically join all servers that run game build 1355 or higher (which should be the majority, if they want players, I guess…)
Red Dead Online is supported on servers with 1355+ game build enforced.
You can find such servers by looking for sv_enforceGameBuild variable. 1355 and above (1436 and etc in the future) will work for Red Dead Online. Also you can ensure game build by looking at DLC Level badge on server’s page.
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