Microsoft has recently released .NET Framework 4.8, which will also be delivered promptly via Windows Update. However, .NET Framework 4.8 should not currently be installed on Exchange Servers. Exchange Server only supports certain .NET Framework versions depending on the update status.
.NET Framework versions not supported by Exchange cause trouble from time to time (see here and here), so you should check which .NET Framework version of Exchange is supported with which CU before installation.
Here is an overview of the .NET Framework versions currently supported by Exchange Server:
Source: Exchange Server Supportability Matrix
To be on the safe side that the .NET Framework version is not updated at some point via Windows Update, a corresponding registry entry can be added:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\WU] "BlockNetFramework48"=dword:00000001
This entry prevents the .NET Framework 4.8 installation. If the "WU" key does not exist, it can be created. Alternatively, the above text snippet can be saved in a .REG file and imported directly into the Windows registry.
Exchange support for newer .NET versions is normally added after some time with CUs for Exchange.
With regard to .NET updates, there is a very interesting article here: