Thanks for your reply,
It confirm what I was thinking but not sure since I’m pretty new to hosting/PHP but I work in IT so learn pretty fast in technology.
One solution that I see would be to develop a package in PHP PEAR package PEPr :: Package Proposals CPanel has a module to install those PEAR package easily for newbie like me 
From my understanding PEAR Packages are derived from PECL packages and luckily there is already a Mcrypt PECL package that is maintained: https://pecl.php.net/package/mcrypt
So it would be to convert this package in PEAR and submit it to the maintainer of PEAR project:
However for the long term it would be better to change InvoicePlane to use the OpenSSL library instead of Mcrypt.
But since I’m new I really don’t know is those suggestions are realist or helpful since I see that Version 2.0 is developed but I don’t know if the release will be soon or in a few years.
If I was using a VPS I could install Mcrypt directly from the PECL packages but I use “shared hosting” so I don’t have access to the linux “root”.
Since you will fix it in the next days I’ve found that I will downgrade to PHP 7.1 in meantime but could you tell me if there is some security issue I should an eye on with PHP 7.1 ?
Thanks for your quick reply 