I went to immigration and have extended my visa and now have an under consideration stamp until the 19th of October.  Does that mean I am legal here after September 26?  What's being under consideration? Basically, as of now I am legal to be here even with an under consideration stamp?

The under consideration stamp allows you to stay in the country until your extension is approved or disapproved (that is rare).

The stamp means your application has been sent to the division headquarter for approval.

 

Why is the immigration giving this type of stamp and not a 30 days stamp straight away? 

 

Thousands and thousands of applications to review and as many persons to stamp.

 

Another amnesty coming? 

1 hour ago, ubonjoe said:

The under consideration stamp allows you to stay in the country until your extension is approved or disapproved (that is rare).

The stamp means your application has been sent to the division headquarter for approval.

 

But I am legal to stay here past September 26?  What if I left Thailand after September 26 and before October without getting the final stamp?

52 minutes ago, JacksSmirkingRevenge said:

But I am legal to stay here past September 26?

Yes

52 minutes ago, JacksSmirkingRevenge said:

What if I left Thailand after September 26 and before October without getting the final stamp?

Immigration on departure from the country could refuse to allow you to leave. Not likely but possible.

1 hour ago, Motmot said:

Why is the immigration giving this type of stamp and not a 30 days stamp straight away? 

 

Thousands and thousands of applications to review and as many persons to stamp.

 

Another amnesty coming? 

The OP's "under consideration" stamp is valid for 30 days from when his current permission to stay is due to expire (seemingly on 19th September), rather than from the date when he applied for the extension at his local immigration office. Nothing to do with the amnesty.