You mean "using utf8mb4_unicode_ci with Antelope file_format" by reducing the index sizes? I don't recommend this. You'd no longer have a standard Moodle site and this could cause issues with subsequent upgrades, e.g. if a Moodle upgrade altered database objects you've modified. Also I think this just won't work. It may work for mdl_analytics_predict_samples from your example but there are tables where the indexes are for unique columns (e.g. mdl_filter_config) where you can't reduce the index size without reducing the column size too. You'd lose data if your data exceeded the smaller size and the now non-standard site could result in support issues which affect only your site.
You don't say why you need to migrate to this server, maybe the decision has been made for you or maybe there's some other issue forcing you down this path but if possible this decision should be reconsidered. The utf8mb4 recommendation was introduced in 2016 with Moodle 3.1. Trying to force a utf8mb4 Moodle site to run on Antelope is a bad idea in my opinion.