Finding information on this is difficult, I’ve built up this table after trawling through patch notes.
This section was last updated 22nd of October 2019
A German version for Skype 2015 is over on MSxFaq’s blog here
Note that the major versions of .Net (ie, 3.x and 4.x) both run side by side on a server

Download Skype for Business for Windows 10 for Windows to enjoy the power of messaging, meetings, and calling features to your mobile device. Skype for Business for Windows 10 has had 1 update. Download the old version of Skype for Mac. If you experience any compatibility issues with Skype for Mac, consider downloading one of the older versions of Skype. MacUpdate stores previous versions of Skype for you since v. Jr岡山駅から後楽園まで片道10分。30分間隔で運行。運賃は100円（小児50円）。35 名着席可能で車椅子対応の大型ワン. Skype, the normal, non-business version is a stock app on Windows 10. Creating a Skype account for the non-business version is easy but settings up Skype for Business is rather complicated. Install Skype for Business. Skype for Business is called as such at present but it still runs under the name of certain apps that Microsoft had before.

Skype for business for 10.10.5 version windows 10

.Net Version	Minimum Lync 2013 CU	Minimum Skype4B 2015 CU	Minimum Skype4B 2019 CU	Note	More Info
Security Update for .NET Framework (3156757) aka May 2017 .Net update (Affects All Versions)	Known Issues	Known Issues	Known Issues	Known Issues	Patch introduces an issue with some clients connecting and web conferencing due to changes in encryption handling. Info and workaround here
.Net 3.5	Any	Any	Any	Required	Required for installation. Lives alongside 4.X
.Net 3.5.1	Any	Any	Any	You may need to repair the .Net 4.x install if you install .Net 3.51 after 4.x	Example issue: https://greiginsydney.com/no-connectivity-with-the-lync-web-app/
.Net 4.0	Out Of Support	Out Of Support	Out Of Support	.Net 4.0 No longer in support	More Info: https://devblogs.microsoft.com/dotnet/support-ending-for-the-net-framework-4-4-5-and-4-5-1/
.Net 4.4	Out Of Support	Out Of Support	Out Of Support	.Net 4.4 No longer in support	More Info: https://devblogs.microsoft.com/dotnet/support-ending-for-the-net-framework-4-4-5-and-4-5-1/
.Net 4.5	Out Of Support	Out Of Support	Out Of Support	.Net 4.5 No longer in support	More Info: https://devblogs.microsoft.com/dotnet/support-ending-for-the-net-framework-4-4-5-and-4-5-1/
.Net 4.5.2	RTM	RTM	Not Supported	Skype4B 2013 CU6 and up require at least this version. Also required for Seafutil/OOS/OWAS	Recommended for new 2013 builds Download here (why are you building a 2013 server?)
.Net 4.6.0	RTM	CU 5 – May 2017	Not Supported	Has security vulnerabilities update to 4.6.2	2015: Conflicting info on this one, best idea is to update to CU5 and .net 4.7.2
.Net 4.6.1	Not Supported	Not Supported	Not Supported	Not Supported on anything! Upgrade to 4.6.2 if possible Otherwise downgrade to 4.6.0	Warning: https://tomtalks.blog/2016/02/dont-install-net-framework-4-6-1-and-skype-for-business-server-2015-lync-server-2013-exchange-server/ Block install: https://support.microsoft.com/en-us/help/3133990/how-to-temporarily-block-the-installation-of-the-net-framework-4-6-1 Removal: http://www.expta.com/2016/02/how-to-uninstall-net-framework-461.html
.Net 4.6.2	CU 4 HF1 – Feb 2017	CU 4 HF1 – Feb 2017	Not Supported	Last supported .Net for Lync 2013	2019: Installer fails https://github.com/MicrosoftDocs/OfficeDocs-SkypeForBusiness/issues/2644
.Net 4.7.0	Not Supported	CU 5 – May 2017	RTM	2015 Ref: https://docs.microsoft.com/en-us/skypeforbusiness/plan-your-deployment/requirements-for-your-environment/server-requirements%C2%A0
.Net 4.7.1	Not Supported	CU 5 – May 2017	RTM
.Net 4.7.2	Not Supported	CU 5 – May 2017	RTM	Download here: https://download.microsoft.com/download/6/E/4/6E48E8AB-DC00-419E-9704-06DD46E5F81D/NDP472-KB4054530-x86-x64-AllOS-ENU.exe
.Net 4.8	Not Supported	CU 9 – June 2021	RTM	Current recommended .Net for Skype4B 2015 and 2019	Download here: https://support.microsoft.com/en-au/help/4503548/microsoft-net-framework-4-8-offline-installer-for-windows 2015 Ref: https://docs.microsoft.com/en-us/skypeforbusiness/plan-your-deployment/requirements-for-your-environment/server-requirements

Server Updates

Skype For Business For 10.10.5 Version 2

Updates for Skype for Business Server 2015 – Always points to the latest update.
Luca Vitali maintains a cache of older updates on his website. Skype4b 2015 and Lync 2013
A list of versions and their build numbers is over on Andrew Morpeth’s blog https://ucgeek.co/lync-updates/

Skype For Business For 10.10.5 Version 7

Article Change Log

Skype For Business For 10.10.5 Version Update

Skype For Business For 10.10.5 Version Windows 10

05/07/2021 – Added .Net 4.8 as supported for Skype4B 2015 CU 9 – Thanks Tobie