MySQL / MariaDB Strict Mode

I was developing some code using MySQL as a DB, with various indexes. I had MySQL running in strict mode.

As an experiment, I switched to MariaDB (also in strict mode) and found that many of indexes were failing because they were too long (above 767 bytes). If I remove Strict mode in MariaDB, the indexes seem to be created.

My question is: are those indexes actually being created, even though they are clearly too large?

If not, how do I deal with indexing VARCHAR strings?