Hi mugilan,
I just tested the choice column in SharePoint list when trying to add an action of update SharePoint Item under Microsoft Flow, and still the column can't display as an available item in the Action pane, this is also true regarding the other "Read-only" columns such as Picture or URL (When formatted as picture), Calculated and the Mnanaged Metadata.
That is hard to say when it would be available. But I suspect this would take a long time.
As the blog shares that:
"
- Modern document libraries will be made available as a data source for PowerApps and Microsoft Flow.
- For Microsoft Flow, you’ll be able to run any flow on demand for a specific document or list item, in addition to waiting for an automated trigger to fire.
"
For more information, please check:
SharePoint welcomes PowerApps and Microsoft Flow
Regards