How can we burn DVD'S now that iDVD has been discontinued?

Go to the App Store and search for "video DVD". You'll find a number of potential candidates to replace iDVD. Most have demo versions you can try before purchasing. If you use both still photos and video files in your product there's only one app I've found that can use both: Wondershare DVD Creator.

As with iDVD 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 the disk image to disk with Disk Utility 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.

As Old Toad says, there are other DVD authoring programs out there. One that has been recommended is DVD Creator by Peng Guiping, for about $14.97 in the online App store. It is not as good as iDVD, but then nothing is. Not quite as easy to use, but reports are that it does produce a movie DVD.

For some helpful information, take a look at the thread in the link below, particularly the post by Batman-15.

https://discussions.apple.com/thread/250714476

-- Rich