Fair question. Dojo based widgets will for sure work in Mendix 10. As MVP I haven’t heard of any date/Mendix version as per when Mendix isn’t supporting Dojo anymore. I do know Mendix has been thinking of releasing a version in which you can choose whether to have the front end run fully in React. To my knowledge there will then still be another version which supports Dojo as well, but you have to actively choose it at the App creation.
Really stopping support for Dojo based widgets will be a very big thing having impact on numerous existing clients, hence will be announced and prepared very well.
So for now, always try to use React based Pluggable widgets, but if a requirement is only met with a Dojo based widget you already have in the project I wouldn’t stress too much. This happens a lot still.