No. Projects created in earlier versions of Vegas Pro can be opened in Vegas Pro 21. However, projects saved in Vegas Pro 21 won't open in earlier Vegas Pro versions (e.g. VP20 and earlier). Please note that the current build of Vegas Pro 21 is build 208.
A few tips:
Any third party FX used in the earlier VP version needs to be compatible with VP21. Most will be (e.g. BCC, NewBlueFX) but there could always be an obscure exception.
.veg projects created in an earlier Vegas Pro version should be copied and renamed for use in a later Vegas Pro version so that the project still remains usable in the earlier VP version if needed.
It's been recommended from time-to-time on the forum over the years to complete an existing project in the earlier version of Vegas Pro and start new projects in the new version.
The above assumes that you have perpetual licences for Vegas Pro and not a subscription.
My Vegas Pro 21 (208) projects keep saving as Vegas Pro 20. Version 20 is still installed. Will removing this fix this or prevent me from opening projects in 21 ?
If you are opening Vegas Pro from a .veg file in Windows Explorer, make sure that you have .veg selected for Vegas Pro 21 as your preferred VP app in Windows Default Apps. It may be that it is currently set for Vegas Pro 20.
@Marcio-VonKruger ... as per my comment just above your new comment, changing the default Vegas Pro version is done in Windows 'Default Apps' - it has nothing to do with Windows Explorer.
In Windows 11, type 'default apps' in the task bar's search field then select that app from the list that opens. In Defalut Apps, type in '.veg' in the search field at the top of the window and then click on the Vegas Pro match that appears. Another window will open showing all the Vegas Pro versions that are installed -just click on the VP21 icon and then click on the 'Set Default' button.