Ubuntu 14.04 LTS ESM, 16.04 LTS ESM, 18.04 LTS, 20.04 LTS, 21.04 have all been fixed and will not be affected by DST Root CA X3 expiry. All letsencrypt servers should continue to work with OpenSSL, GnuTLS, web-browsers, with either default long chains (default config), or the short CA chains.
Make sure all updates are installed and carry on.
For 14.04 & 16.04 one does need ESM enabled to get access to these updates.