I think you can simplify this whole EntryType stuff, if you want too look up an enum key by the string value:
StandardEntryType.valueOf(...)
Or alternatively, use the guava equivalent https://guava.dev/releases/19.0/api/docs/index.html?com/google/common/base/Enums.html

The problem here is that the entry of StandardEntryType uses big camelcase nomenclature, I can't directly use StandardEntryType.valueOf(...), or I need to find a good way from JabRef to deal with big camelcase format and all-caps dashed separation format conversion.

And some types of naming do not match exactly. For example, the API returns PAPER_CONFERENCE corresponding to the CONFERENCE of StandardEntryType.

Ah I understand, however, you can directly operate on the Enum, there is the possibility to use an EnumSet and you have an Optional to check for the type.
This approach is simpler and you don't need to create a new Map with the keys and values

       Optional<StandardEntryType> foundType  = EnumSet.allOf(StandardEntryType.class).stream().filter(type-> type.getDisplayName().equals(strType)).findAny(); 

Optional<StandardEntryType> foundType  = EnumSet.allOf(StandardEntryType.class).stream().filter(type-> type.getDisplayName().equals(strType)).findAny(); 

This is invalid if the input string is ARTICLE

New version~

private StandardEntryType typeStrToEnum(String typeStr) {
    StandardEntryType type;
    if ("PAPER_CONFERENCE".equals(typeStr)) {
        type = StandardEntryType.Conference;
    } else {
        String upperUnderscoreTyeStr = CaseFormat.UPPER_UNDERSCORE.to(CaseFormat.UPPER_CAMEL, typeStr);
        type = Enums.getIfPresent(StandardEntryType.class, upperUnderscoreTyeStr).or(StandardEntryType.Article);
    }
    return type;
}

Just remove the disabled lines

fixed.

May I ask whether you committed that change?

I have deleted these two lines

// see https://github.com/JabRef/jabref/issues/5804 
// set.add(new ACMPortalFetcher(importFormatPreferences));

do you mean I still need to delete this line?

// set.add(new GoogleScholar(importFormatPreferences));

But this is another Fetcher

Sorry, was too late. Everything allright! 🎉

May I ask whether you committed that change?