Improvement is the overall AcyMailing internal coding and code structure. Majority of v5 features has been implemented in v10. We didn't implement some minor features, which in the past few years no one asked for.
The feature you are talking about here is one of them. You are the very first person in a few years to ask for it. The developers must focus on important things and other more important features, and let some less attractive feature to pass on.
There is always a workaround for your case. Export required users, create one custom field which will contain a flag (0 or 1 for example) and will represent the subscriber to whom you want to send newsletter, import back the same subscribers with this custom field value included. After that, use the Segment feature and include only those subscribers who have this flag enabled.
