Since other planets have their own gravitational pull, I'm wondering if their effect on Earth's orbit has ever been measured? I know that in comparison with the Sun's pull the effect might be rather small, but I'm wondering if it's measurable...
For decades, geologists have understood that climate changes are due in part to gradual shifts in the Earth’s orbit, which are caused primarily by Venus for its proximity and Jupiter for its mass, and repeat regularly every 405,000 years. In 2018 a team of geologists and Earth scientists unearthed the first evidence of these changes – sediments and rock core samples that provide a geological record of how and when these changes took place.
https://www.pnas.org/content/115/24/6153 "Empirical evidence for stability of the 405-kiloyear Jupiter–Venus eccentricity cycle over hundreds of millions of years"
So given the timescales involved, this is as close to measuring this effect as one could expect to get.
Has the effect of other planets on Earth's orbit been ever measured?
...but I'm wondering if it's measurable...
set up the question in an interesting way because it calls into question (intentionally or not) what kind of measurement would satisfy the question.
The motion of the Earth and other planets can be numerically reproduced with incredible accuracy, and this can only be done when taking into account a large number of planet-planet gravitational effects. Here's an example of a python script doing an approximate calculation and being compared to data from the Horizons web interface which uses data from the JPL Development ephemerides or DE's which can reproduce the motion of the inner planets to kilometers and outer planets to tens of kilometers.
After the Sun, Jupiter's effect is huge, but interaction between the inner planets is quite big as well, with Venus's and Earth's effects upon each other being quite important. Venus also has a big effect on Mercury as well.
In addition to affecting the inner planets directly, the massive outer planets Jupiter through Neptune move the Sun around by a million kilometers as well, and the inner planets will track the Sun's motion slow movements on the scale of decades or centuries.
But besides showing that simulations match measurements, I don't think there has been any way to perform an actual measurement of the effect of one Planet on another. So the answer to your question could be yes or no depending on how you would like to think of a measurement.