JabRef 5.3--2021-05-15--0a26d31
Linux 4.19.0-16-amd64 amd64
Java 16.0.1
JavaFX 16+8

• Mandatory: I have tested the latest development version from http://builds.jabref.org/main and the problem persists

For a library in BibTeX mode, if an entry of type @Article contains the field month, JabRef displays it in the tab Deprecated fields. In BibTeX, the field month is an optional field (see http://mirrors.ctan.org/biblio/bibtex/base/btxdoc.pdf on page 8). Hence, it should not be displayed as deprecated.
My guess is that the deprecated fields are displayed as if the library was in biblatex mode. And I believe the concept of deprecated field does not apply to BibTeX fields. Hence, the tab Deprecated fields should not even be displayed in BibTeX mode.

Steps to reproduce the behavior:

1. Create a bib file containing:

@Article{ostgaard2002,
  author = {Foo Bar},
  title =  {energy},
  year =   {2012},
  month = sep,
}

2. open it with JabRef (in BibTeX mode)
3. open the entry editor for this entry
   ==> The tab Deprecated fields is displayed and contains the field Month.

• Same in the entry type @book for the field address (it is an optional field).
• Same in the entry types @phdthesis, @inproceedings, and @techreport for the fields address and month (they are optional fields).
• Same in the entry type @booklet and @manual for the fields address, month and year (they are an optional fields).
• and other cases