From puppet 7 legacy functions are no longer supported as such we should either drop them if they are no longer needed to update them to use either a puppet function or the newer ruby API https://puppet.com/docs/puppet/latest/functions_refactor_legacy.html.
From a quick search the following are legacy functions
modules/admin/lib/puppet/parser/functions/add_all_users.rb
modules/admin/lib/puppet/parser/functions/unique_users.rb
modules/graphite/lib/puppet/parser/functions/configparser_format.rb
modules/labstore/lib/puppet/parser/functions/mount_nfs_volume.rb
modules/librenms/lib/puppet/parser/functions/phpdump.rb
modules/mediawiki/lib/puppet/parser/functions/compile_redirects.rb
modules/network/lib/puppet/parser/functions/slice_network_constants.rb
modules/profile/lib/puppet/parser/functions/nutcracker_pools.rb
modules/redis/lib/puppet/parser/functions/redis_get_instances.rb
modules/redis/lib/puppet/parser/functions/redis_shard_hosts.rb
modules/role/lib/puppet/parser/functions/kafka_cluster_name.rb
modules/role/lib/puppet/parser/functions/kafka_config.rb
modules/service/lib/puppet/parser/functions/merge_config.rb
modules/wmflib/lib/puppet/parser/functions/cron_splay.rb
modules/wmflib/lib/puppet/parser/functions/ensure_mounted.rb
modules/wmflib/lib/puppet/parser/functions/get_clusters.rb
modules/wmflib/lib/puppet/parser/functions/htpasswd.rb
modules/wmflib/lib/puppet/parser/functions/init_template.rb
modules/wmflib/lib/puppet/parser/functions/ipresolve.rb
modules/wmflib/lib/puppet/parser/functions/puppet_ssldir.rb
modules/wmflib/lib/puppet/parser/functions/requires_realm.rb
modules/wmflib/lib/puppet/parser/functions/secret.rb
modules/wmflib/lib/puppet/parser/functions/ssl_ciphersuite.rb
modules/wmflib/lib/puppet/parser/functions/systemd_template.rb