The libmcrypt library libmcrypt abandoned in 2007.
Last year, PHP developers accepted the proposal to declare the mcrypt extension obsolete due to the fact that the core library has errors and is not supported, but also because of a rather poorly designed set of functions that contribute to the appearance of errors in cryptography. Sometime later, the extension will be moved into the PECL package and will no longer be supported.
Regarding your phpmyadmin - the latest release of 4.6.5.2 was released on December 5, an error was noticed and corrected only on December 10 . Tritely, the phpmyadmin community missed the preparation of release notes released on December 1, PHP 7.1, and no one even tested compatibility during alpha and beta tests, or release candidates.
Pay attention to the committer's comment, by the way:
There is a lot of problems in PHP 7.1 and 4.6 right now.
Apparently, other problems are known to work phpmyadmin 4.6 under PHP 7.1.
mcrypt_list_algorithms? because there are vulnerabilities and there are more reliable functions, it’s logical ......... outdated with PHP 7.1 - Alexey ShimanskyNotice.... well, or those that areE_DEPRECATED- Alexey Shimansky