What the message says is that iDVD will not be compatible with future Mac systems.
It does run on Mojave but some have problems with some themes and functions. I find that if I use an older, non animated theme with no drop zones I have the best chance of creating a good video DVD project.
Follow this workflow to help assure the best qualty video DVD:
Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.
To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.
Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.
If you use both still photos and video files in your video DVD then there's a 3rd party app that will do both: DVD Maker Lite - DVD Creator. It too can create a disk image of the project to separate encoding from the burning process.
You can download the free(demo) version of it to experiment with it to see if it will do what you want. It's GUI is nowhere as easy to learn as iDVD but it will do the job. The demo version puts a watermark on the presentations which is removed in the purchased version.
If you only use video files in your video DVD projects there are several apps in the App Store that work quite well. Some have demos or free versions.
