Telegram 8.3 provides CONTENT PROTECTION feature.
Some details are available in official Telegram blog here.

What is it?
Now group/channel owner can protect content of his group/channel by restricting some actions for all group members or channel subscribers.

What can be restricted?
1. Group (or channel) owner is the only one who can protect content. Admins can not.
2. Group owner can restrict the following actions:
• forwarding messages;
• saving messages to "Favorite";
• saving media to device.
3. Group owner can restrict only all of these actions, not some of them.
4. Group owner can restrict there actions only for all group members / channel subscribers together (including all admins regardless their permissions), not for some of them.
5. Group owner can restrict these actions only for all group messages, not for some of them. If the message was sent before group was protected then that messages can not be forwarded/saved too.

How to protect my content?
1. Open group.
2. Tap on group title - pencil icon - "Group type"
3. Turn on "Restrict saving content" switcher at the bottom of the screen.

Does Plus Messenger have any specialties?
Plus Messenger 8.3.1 allows saving content from protected groups/channels to "Favorite". But not forwarding or saving media to device.

Why can not I open a group or a channel?
If you get the message "Group administrator has restricted content from the group. Update your app to open the chat" then
1. Group or channel is protected by its owner.
2. You use outdated app version.

You need to install 8.3 (or newer) version of your app: read details below.

How to get Plus Messenger 8.3?
Read here: https://t.me/plusmsgrfaq/23

Any ways to copy/forward content of protected groups/channels?
No. It breaks Telegram Terms of Service.