This issue was addressed with improved checks. This issue is fixed in watchOS 5.3. Users removed from an iMessage conversation may still be able to alter state.
[
{
"product": "watchOS",
"vendor": "Apple",
"versions": [
{
"lessThan": "watchOS 5.3",
"status": "affected",
"version": "unspecified",
"versionType": "custom"
}
]
}
]