The 2021 U.S. Open cup will happen but in a reduced format. There will be only 8 MLS teams, so its very possible that we don't take part in it ( since I assume they will go with MLS CUP winners/ finalists and teams making it farther into the playoffs or better record in the overall table).
Summary Below:
May 4-5 - First Round
- Four amateur teams from the Open division comprising USL League Two, NPSL and local qualifying tournaments (from among 36 eligible 2020 qualifiers)
- Four teams from USL League One and NISA (two each)
- Eight teams from USL Championship
May 18-19 - Round of 16
- Eight teams from MLS
May 25-26 - Quarterfinals
June 15-16 - Semifinals
June 29-30 - Final
https://www.si.com/soccer/2021/02/08/us-open-cup-2021-plans-us-soccer-teams-format
Summary Below:
May 4-5 - First Round
- Four amateur teams from the Open division comprising USL League Two, NPSL and local qualifying tournaments (from among 36 eligible 2020 qualifiers)
- Four teams from USL League One and NISA (two each)
- Eight teams from USL Championship
May 18-19 - Round of 16
- Eight teams from MLS
May 25-26 - Quarterfinals
June 15-16 - Semifinals
June 29-30 - Final
https://www.si.com/soccer/2021/02/08/us-open-cup-2021-plans-us-soccer-teams-format