Rasmus, I am just a user, and I feel as enraged as anyone who comes here sometimes. Ai is a flawed tower of bugs, and you don’t have to remind me of it :)

What I see is that you have Construction Guides disabled. These apply when you draw a NEW object (according to the manual, https://helpx.adobe.com/illustrator/using/rulers-grids-guides-crop-marks.html)
What you see when you edit a path like you showed previously, are guides for Transform Tools — since you transform the path. If you uncheck the Transform Tools option, you won’t see 45° angles.

Why can’t we customize angles for object guides? Well... there are related requests, but it seems not that much people upvote them:
1. More fields to Smart Guides angles — http://illustrator.uservoice.com/forums/333657/suggestions/48960053
2. True customizable drawing 'angles' for all tools when using SHIFT key (pen, pencil, brush) — http://illustrator.uservoice.com/forums/333657/suggestions/48073562
3. Smart Guides and the Rotate Tool: Incremental Rotation Angles — http://illustrator.uservoice.com/forums/333657/suggestions/39621259

Egor, thank you for the condescending nonsense! I understand how great it must feel, to be so much better than your paying users. Why even bother to consider ones own flaws, just because users are complaining on every social media platform?

Here is a screenshot of my smart guide settings. Notice how the 45° and 90° smart guides are turned off. Like they always have been. But the feature is turned on regardless. Nothing new. Eurasia has always been at war with Oceania.

Why even have settings, when you know what people want? And why have bug reports, when a bug is something that has never happened?

Rasmus, the fix works, because it restores the previous behavior.
The extension of curved segments never worked in the first place, so we can’t expect from a 'fix' to introduce it :)
And 45° and 90° smart guides are nothing 'new' and were there for much longer, before 'Line Extension' even happened...

But if you wish for a 'Line Extension' to be a general 'Path Extension' (something it never was, and is a good thing to have), you can upvote a corresponding feature request here: http://illustrator.uservoice.com/forums/333657/suggestions/39843283

This fix doesn't work. You can extend straight lines, but any curve along any part of a line renders the fix useless. But the new (useless) 45° and 90° smart guides are there.

Did anybody need a smart guide 45° degrees from the origin, rather than the line extension we've had for years?

In update 29.0.01, everything really works, you can update! Thanks for fixing the problem!

The most recent update (29.0.01) fixes this issue thank you!

Rama / Adobe – It's really not good enough to update software with such carelessness, when users are paying such a high premium.

In previous versions a path at any angle can be extended and restrained to same angle with Smart Guide, this is not the case with 2025 version.

Yes this needs to be reverted. For professional software to lose this functionality is awful.

The proposed fix is not good and does not match the behavior we had before.

The tolerance the guide uses to allow the sliding is MUCH smaller with this fix than it was before.
Take a look at these two GIFs attached: one is from Ai CC2018 (v22.0.0), when the guide is stable and the tolerance allows to move a point almost continuously, and the other one from PR v29.0.1.192, with the guide blinking, and precise sliding required.

Not fully fixed.

We need this fixed ASAP.

This is not good, please fix this Adobe!

Same for me. In the meantime for everyone that doesn't know how to extend a line in the same angle without this tool, just add a guide over the line and extend it.

I literally cant do my job without this. Eyeballing stuff is so unprofessional. Been following every update religiously waiting for this fix. Even switched from beta back to normal illustrator, no difference.
Glad it's not just me. Downgrading till it's fixed.

I use Line Extension regularly. Please bring this function back! Thank you.

Makeshift solution from up until a few years ago: You can uniform-scale a line from the next anchor point, to do so, select the segment, hit scale, select the anchor point as your base to scale from, drag the second anchor, while keeping "Uniform" displayed.

I need the Line Extension function as I use it on a daily basis, it is not very efficient trying to eyeball the position of an extended line. That's why I don't use Corel as they don't have these kind of functions.

Thank you for looking into this 🙏

This is one of my favourite features about smart guides since there is no other easy way to do this.

I, too, use this feature daily. All work-arounds are super time-consuming. Please fix this bug!