Group Replication has been around for some time now and the feedback from community is overwhelming. One of the suggestions (BUG#84795) received is related to deploying a mechanism to prevent a server from being updated after stopping group replication. Without this mechanism the server can continue to receive update transactions, which can then committed locally only, very likely not meeting the expectations of the user.
To improve safeness we changed the behavior, in MySQL 8.0.2. When executing STOP GROUP_REPLICATION it enables super read only mode on the server. This will cause the server to be read only by default when it leaves the group, regardless of the reason it has left.
In addition,on MySQL 5.7.19 and 8.0.2, now it is also possible to protect the writes from the beginning, that is, user can start MySQL with super_read_only= 1 (thank you Kenny for BUG#84728 and BUG#84733). Once Group Replication starts successfully, it will adjust super_read_only properly. On multi-primary mode, the read mode will be unset on all joining members; On single-primary, only on the primary. If by any unlikely reason, the Group Replication start fails, the read only mode is preserved, thence no writes are allowed.
Download the new MySQL 8.0.2 DMR and check it out. As usual, lets us know your feedback, e.g., through bug reports, feature requests, the replication mailing list or even through comments on this or other blogs on this website. Mind you that MySQL 8.0.1 is a development milestone release (DMR), thence not declared generally available yet. Use it at your own risk.