Is the standard Canon 18-55 lens the same as 5 years ago?

It depends upon which "standard Canon 18-55 lens" it is to which you are referring.

Currently there are still kits offered new in many places that contain the EF-S 18-55mm f/3.5-5.6 IS II that you bought with your 600D as early as the first quarter of 2011. Any kit that contains this lens contains a lens built to the same exact specifications as the lens you already own. Any improvements Canon makes to a lens would result in a new name for the updated lens. No such updated lens has been announced as of 10/01/2016.

There are also many kits currently offered with the newer EF-S 18-55mm f/3.5-5.6 IS STM that was introduced in 2013.

Have these lenses improved significantly over that timescale?

It depends upon what you would consider "significantly improved."

The newer STM version of the 18-55mm is a new optical design with 13 elements in 11 groups compared to your IS II lens with 11 elements in 9 groups. It has a new focus motor designed to be smoother and quieter for use when recording video. The STM version also focuses faster than the older IS II. There were also many improvements made that go beyond optical an AF performance.

In Bryan Carnathan's review of the EF-S 18-55mm f/3.5-5.6 IS STM at The-Digital-Picture he compares the two lenses and lists the following features included in the newer STM model:

Here are some of the differences between the 18-55 IS II and the 18-55mm IS STM Lens:

STM (Stepping Motor) AF (vs. Micro Motor)
Continuous AF during video recording with compatible cameras (those with Hybrid CMOS AF)
Vastly improved manual focus ring
Front element does not rotate and does not extend with focusing
Full time manual focusing supported during shutter release half-press
Shorter minimum focus distance (9.8” vs. 11", 250mm vs. 280mm) and maximum magnification (0.36x vs. 0.28x) specs (read about reality below)
New optical design with 13 lenses in 11 groups (vs. 11/9)
7 rounded aperture blades (vs. 6 rounded)
Larger zoom ring
New petal-style lens hood that provides up to 0.46" (11.7mm) better protection from light and damage.
50% higher price (in kit)

The optical differences between the two lenses are a bit of a mixed bag. One is slightly better at some particular focal lengths and apertures. The other is slightly better at other particular focal lengths and apertures. They are both budget kit lenses and neither will be mistaken for a premium lens. At the same time, many people find them useful for the flexibility they offer at a bargain price and both are better optically than the older non-IS versions of the 18-55mm kit lenses sold before 2007. The optical design of the older 18-55mm lenses wasn't much, if any, different - but the quality control during production was and many side-by-side tests by reviewers and testers bear this out.