Is there a good resource to identify all conference committees that have already occurred in the current Congress? If not, is there a good resource for past Congresses? (By past Congresses I mean not the 116th Congress).
No, at least not that I could find. However, if the conference committee produced a report, the report is listed at United States Congress Conference Reports.
National Defense Authorization Act for Fiscal Year 2020
Congress:116th Congress 1st Session Report Number:116-333
Date:12/10/2019 8:05:00 AM Bill Number:S. 1790
Consolidated Appropriations Act, 2019
Congress:116th Congress 1st Session Report Number:116-9
Date:2/14/2019 8:32:17 AM Bill Number:H.J. Res. 31
Agriculture Improvement Act of 2018
Congress:115th Congress 2nd Session Report Number:115-1072
Date:12/11/2018 11:10:00 AM Bill Number:H.R. 2
Department of Defense, Labor, Health and Human Services, and Education Appropriations Act, 2019, and Continuing Appropriations Act, 2019
Congress:115th Congress 2nd Session Report Number:115-952
Date:9/14/2018 1:20:00 PM Bill Number:H.R. 6157
There is an API for accessing information but I am not familiar with what is available. See New govinfo API.