The issue was addressed with improved deletion. This issue is fixed in iOS 13.4 and iPadOS 13.4. Deleted messages groups may still be suggested as an autocompletion.
[
{
"product": "iOS",
"vendor": "Apple",
"versions": [
{
"lessThan": "iOS 13.4 and iPadOS 13.4",
"status": "affected",
"version": "unspecified",
"versionType": "custom"
}
]
}
]