This is not true, in the email I received they advise to save the old version somewhere in case you need it.

FifthAs of May 31, 2023, Synology will no longer allow access to installation files for some older versions of DiskStation Manager (DSM), packages, firmware, clients, and Synology Router Manager (SRM) due to changes in license agreements.

This change may affect owners of older Synology systems and legacy Synology systems (with End of Life or EOL status).

These include:

devices that are not yet set up,
devices that need to be reinstalled,
Devices that need to be updated to the latest software.

Note: This does not affect devices that are still in use.

The software versions below will be withdrawn from our website and distribution effective May 31, 2023.

It concerns the following products
NAS/SAN (DSM) 6.2.3 and below
DSM UC 3.1.1-23036 and lower
Video Station 3.0.2 and below
Sound Station 7.0.0 and below
Networking (SRM) 1.0 – 1.1.6 and 1.2 – 1.2.4
Media Server 1.4 and below
Monitoring station 8.2.8 and below
Monitoring Station Client 2.1.0 and below
Specific to the VisualStation model

Recommendations

We recommend that you upgrade your device(s) to the latest DSM and package releases.

For devices that cannot be upgraded to DSM 6.2, we recommend that you keep a copy of your device’s installation packages and files in case a reinstallation is required in the future. Go to the download center

Note: You are receiving this email because you have older and/or affected devices registered to your Synology account.