555 timer circuit causes spikes on power supply line - how to decouple?
Your spikes have at least THREE possible causes

where you tie the Scope Probe GROUND lead (but you can also HEAR this)

lots of wiring inductance; you can reduce this, by using TWISTED PAIRS to provide Hot/Return paths

no local provision for charge directly where needed; this requires a SHUNT capacitor

no series impedance (inductor or resistor) in the VDD path, to modestly separate the charge demands of module_A from the charge demands of module_B;
without this series impedance, a charge transient ANYWHERE will cause glitches EVERYWHERE. Yes, if you use a resistor, you will eat up headroom; if you use an inductor, you may get ringing unless you dampen (by parallel resistor) the LC filter.

not using &quot;Local Battery&quot; concept, where a large shunt capacitor placed across the 555/LED (100uF? 10uF? 1uF), AND a moderate SERIES impedance in the VDD from the 555/LED to your global raw VDD (which should have its own 10uF cap) create a Low Pass Filter (actually a PI filter, what with that global 10UF )

have VERY SHORT leads on the shunting capacitor (and  very short PCB traces);

do not have a Global VDD plane to each IC; that simply ensures the ringing/glitching/spikes of one IC will become ringing/glitching/spikes on VDD of ALL ICs

you may need to create a private island (a 1&quot; square of copper) VDD region, near your IC/load, and use VERY SHORT leads (surface mount capacitors, to the underlying Ground plane.


Learn, and respect, this: Vbounce = L * dI/dT
which means 10nanoHenry (10mm of wire) with dI/dT == 100mA in 10 nanoSeconds,
will cause

Vbounce = 10nH * 0.1 amp / 10nSec = 0.1 volt bounce,

with strong ability to upset all circuits around
YES you get to design a filter for your VDD.
YES you get to design the VDD distribution system; if you want microvolt (or nanoVolt -- its possible) clean/quiet systems, that ideal abstracted IC performance requires massive VDD filtering and planning, because the IC's PSRR not only is finite even at DC, but rapidly degrades at high frequencies, to become about Zero dB at 1MHz for every IC;