What regulation do you think would apply? And how/why would this regulation differ for e2e encrypted chat products like Signal, Telegram, WhatsApp etc that can’t access text based chat messages?
IANAL, but the enterprise companies that make up Slack's customer base are often under regulations to preserve their employees' official communications in case they are needed for future investigations. Those same regulations prevent them from using the products you listed as official communication channels.
FINRA recordkeeping and retention requirements, as well as SEC statute around records and reporting requirements (finance industry specific).
Companies that have such audit requirements do not use Signal et al either for internal comms.