InDesign Back compatibility

[Jrussial]

I assume InDesign 2020 in Creative Cloud is not completely compatible with the 2019 version. If it works as previous updates have, a file created in 2019 can be opened in 2020, but a file created or saved in 2020 probably won't open in 2019. Is there a way to save in 2020 (other than saving to PDF) that will allow a file to be saved in a format that can be opened in 2019. 

[Frans v.d. Geest · Accepted solution]

Yes, save as idml.