Hi,
it depends on which version you did upgrade your vCenter Server to. vSphere Client (HTML5) was under development when 6.5 version was released.
https://kb.vmware.com/s/article/2147929
In one of the updates to vSphere 6.7, vSphere Client was considered as complete. Now it is still developed and VMware adds new features to it.
So I suggest you should consider to upgrade your environment to the latest vSphere version.
Regards
Radek
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