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When descending on an ILS you can descend to 100' above the TDZE if you have the approach lighting system in sight. Cody Johnson wrote an article explaining how one pilot mis-interpreted what the rules allowed him to do, descended below 100' when there were obstructions in the way and crashed into a hill. (https://iflyamerica.org/misunderstanging_part91.asp )

Section 91.129(e)(3), Operations in Class D Airspace, is also extremely applicable in this accident. It says, 'An airplane approaching to land on a runway served by a visual approach slope indicator shall maintain an altitude at or above the glide slope until a lower altitude is necessary for safe landing.'

Class C and B incorporate Section 91.129(e)(3) by reference, so the rule applies in that airspace as well. It doesn’t specifically apply to operations at airports in Class E airspace.

My question is are there any ILS approaches in Class E airspace? Is there a a rule that prohibits them, or conversely, only allows them when there is a tower?

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There are plenty of ILS approaches in Class E surface area (e.g. KCLM, KHQM, KPWT). In fact the presence of an ILS will usually require that some kind of controlled airspace extends to the surface. For an untowered airport this means a Class E surface area.

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Visalia, CA (KVIS) is an uncontrolled airport with a Class E surface area and an ILS with 200' minimums.

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    $\begingroup$ ILS approaches are even possible in Airspace G. $\endgroup$
    – pcfreakxx
    Feb 24, 2017 at 11:36
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    $\begingroup$ @pcfreakxx please name one. I ask because the FAA has ruled that IFR is Class G is illegal (see faa.gov/about/office_org/headquarters_offices/agc/…) $\endgroup$
    – Steve Kuo
    Feb 24, 2017 at 22:25
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    $\begingroup$ @SteveKuo -- no, IFR in G without a clearance is illegal. $\endgroup$
    – UnrecognizedFallingObject
    Feb 25, 2017 at 1:01
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    $\begingroup$ @SteveKuo Actually you need a RMZ (or airspace F). One example is EDGS aerodrome. $\endgroup$
    – pcfreakxx
    Feb 25, 2017 at 8:27
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    $\begingroup$ @TomMcW But they do exist according to ICAO $\endgroup$
    – pcfreakxx
    Feb 25, 2017 at 19:49
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Around airports, instrument approaches are what class E is all about. That's what results in the "keyhole" shape of class E 700ft and surface area extensions.

Class E surface area surrounding an airport is usually used for airports that do not have a tower or at times when the tower is not operating.

From AIM 3-2-6 §e.1. Surface area designated for an airport where a control tower is not in operation. Class E surface areas extend upward from the surface to a designated altitude, or to the adjacent or overlying controlled airspace. The airspace will be configured to contain all instrument procedures.

(a) To qualify for a Class E surface area, the airport must have weather observation and reporting capability, and communications capability must exist with aircraft down to the runway surface.1

(b) A Class E surface area may also be designated to accommodate part-time operations at a Class C or Class D airspace location (for example, those periods when the control tower is not in operation).

But Class E extensions are also used to protect approaches to towered airports in B,C, or D areas.

AIM 3-2-6 §e.2. Extension to a surface area. Class E airspace may be designated as extensions to Class B, Class C, Class D, and Class E surface areas. Class E airspace extensions begin at the surface and extend up to the overlying controlled airspace. The extensions provide controlled airspace to contain standard instrument approach procedures without imposing a communications requirement on pilots operating under VFR. Surface area arrival extensions become part of the surface area and are in effect during the same times as the surface area.1

Anywhere there are instrument approaches not protected by class B, C, or D airspace you can expect a class E extension. Anywhere aircraft on approach are expected to go below 1000 feet there will usually be a class E surface extension.2

1FAA Aeronautical Information Manual[PDF]

2 Russ Roslewski, How are Class E surface area extensions determined?

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KPNE has an ILS for their 24 runway they are class D airspace between 1100-0400Z and class E all other times. The approach, as far as I know, remains on while the airport is class E.

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