How to get a Youtube channel RSS feed after 2015 April 20 (without v3 API)?

At RSS Reader section https://support.google.com/youtube/answer/6098135?hl=en there is an option to export to an OPML file your subscriptions. Then, looking at the contents of the OPML you can extract the feeds, and the structure of each feed is:

https://www.youtube.com/feeds/videos.xml?channel_id=XXXX

So you could generate new feeds from this structure if you know the channel id. This kind of feeds are not getting the "https://youtube.com/devicesupport" error, so I expect they are going to keep working.

You can get the feeds like this:

https://www.youtube.com/feeds/videos.xml?channel_id=CHANNELID
https://www.youtube.com/feeds/videos.xml?user=USERNAME
https://www.youtube.com/feeds/videos.xml?playlist_id=YOUR_YOUTUBE_PLAYLIST_NUMBER

But the JSON format which used to be supported (with additional parameter &alt=JSON) is not supported anymore.

Additionally you can request for API key for public access to your YouTube videos from your developer console and get YouTube Videos, Playlists in JSON format like this:

- Get Channels: 
  https://www.googleapis.com/youtube/v3/channels?part=snippet%2CcontentDetails&forUsername={YOUR_USER_NAME}&key={YOUR_API_KEY}
- Get Playlists: 
  https://www.googleapis.com/youtube/v3/playlists?part=snippet%2CcontentDetails&channelId={YOUR_CHANNEL_ID}&key={YOUR_API_KEY}
- Get Playlist Videos: 
  https://www.googleapis.com/youtube/v3/playlistItems?part=snippet%2CcontentDetails%2Cstatus&playlistId={YOUR_PLAYLIST_ID}&key={YOUR_API_KEY}

More information from YouTube v3 docs.