I don't remember ever seeing a "send all" button.

What would it do?

Please show us a link to your reference to this.

Sorry, I meant 'Reply to all'. The button has been removed.

It's been replaced, by default, by a "Smart Reply" button that is supposed to adapt itself to the circumstances.

I don't have any use for it myself, as I change the appearance to place all the controls which I consider useful in the main toolbar. You should be able to customize all or any of the Toolbars and add or replace controls to suit your taste.

Right-click in empty space on the Toolbar you want to change and select "Customize". You should find various reply button options in the panel that appears, and you can drag and drop into the offending toolbar. Note that each toolbar has its own customization pane, and it's generally not helpful to try to drag controls onto a different toolbar. You can also remove unwanted controls by dragging them to the customization pane, and you can rearrange the tools by dragging them along their toolbar.

I do not even have the 'smart reply' as an option when I try to customise. I have re-loaded Thunderbird but I still do not seem to be able to customise the toolbar.