gmail sign-in bounced by Google

Why does google block sign-in to gmail from em client? Google says that its an insecure app not using best security technology. This only started recently after years of untroubled use of em client. Something changed?

This depends on how you are connecting to Google. If you setup your Gmail account in eM Client using Automatic Setup, you will be connected using OAuth security, which is the security policy favoured by Google. In that setup, less secure apps can be disabled in Google settings. 

If you setup your account manually, using Mail > Other, you will probably not be using OAuth, so less secure apps may need to be enabled in Google settings. If you are using POP3, you have to enable less secure apps.

There is also some misunderstanding of the notifications sent by Google. Sometimes users interpret what was sent as saying they are using a less secure app, when in fact all Google is saying is that you are using a low risk app with access to your data. Be sure you have understood what has been written.

For an example of such a misunderstanding, please see https://forum.emclient.com/emclient/topics/em-client-is-not-following-the-google-new-security-standa… 

That is probably not the case with you because it does not block access to Google, just suggests that you review what is connecting.